FIGS. 1 and 2 illustrate a conventional rail structure 12 for a drawer 11. The rail structure 12 mainly includes a pair of guide rails 13 for fixedly mounting to predetermined positions on a piece of furniture 10, and a pair of slide rails 14 for fixedly mounting to predetermined positions on two side panels of the drawer 11. Each of the guide rails 13 is provided at a front end with a first roller 15, and each of the slide rails 14 is provided at an upper edge with an inward stop plate 17, and at a rear end with a second roller 16. The second rollers 16 on the slide rails 14 are adapted to rotate along the guide rails 13 for the drawer 11 to move in correct directions, and the stop plates 17 are adapted to press against tops of the first rollers 15, so that the drawer 11 may slide smoothly. The drawer 11 includes a front panel 18 that has a width larger than an overall width of an inlet on the furniture 10 for the drawer 11, so that the rail structure 12 could not be seen behind the front panel 18 when the drawer 11 is in a closed position. The front panel 18 also prevents the drawer 11 from being completely pushed into the inlet. Usually, the front panel 18 has a vertical position decided according to heights of outer ends of the stop plates 17 relative to the front panel 18. In the event the front panel 18 has two lateral ends located at different heights, a common solution is to dismount and remount the guide rails 13 to corrected positions. When the height difference between the two lateral ends of the front panel 18 is small, it might be neglected or ignored by a worker to save the trouble of remounting the guide rails 13. And, in the event the guide rails 13 could not be adjusted to desired height or inclination, the slide rails 14 must be dismounted and remounted to suitable positions for compensating errors in the positions of the guide rails 13.